Mechanism of current leakage in Ni Schottky diodes on cubic GaN and Al x Ga 1-x N epilayers

Ni Schottky-diodes (SDs) 300 μm in diameter were fabricated by thermal evaporation using contact lithography on cubic GaN and Al x Ga 1-x N epilayers. Phase-pure cubic GaN and c-Al 0.3 Ga 0.7 N/GaN structures were grown by plasma assisted molecular beam epitaxy (MBE) on 200 µm thick free-standing 3C-SiC (100) substrates. The quality of the cubic group III-nitride epilayers was checked by high resolution X-ray diffractometry, atomic force microscopy and photoluminescence at room temperature and at 2 K. Large deviations from the thermionic emission transport were observed in the current voltage (I–V) behavior of these SDs. Detailed analysis of the I–V characteristics at 300 K and at low temperature showed that a thin surface barrier is formed at the Ni semiconductor interface. Thermal annealing in air at 200°C alters the composition of this thin surface barrier and reduces the leakage current by three orders of magnitude. The doping density dependence of breakdown voltages derived from the reverse breakdown voltage characteristics of c-GaN SDs is in good agreement with theoretically calculated values and follows the expected trend. From these experimental data a blocking voltage of higher than 600V is extrapolated for c-GaN films with a doping level of N D = 5×10 15 cm −3 .